Add self-service event creation from nodes#show

nodes#show's events table now renders unconditionally (previously
hidden entirely for a zero-event node) and gains an "add event" link.
The suggested tag_list is derived from the page's own category tags
via NodesHelper::DEFAULT_EVENT_TAG_BY_PAGE_TAG (erfa-detail/
chaostreff-detail -> open-day) rather than hardcoded, and degrades to
a blank field for any node whose tags don't match - deliberately
universal, not chapter-specific, since Updates have historically
carried event dates the same way.

events#new surfaces *why* the tag was pre-filled via flash.now (not
flash - this is a same-request render, not a redirect), using an
explicit auto_tag_source param passed alongside tag_list rather than
having the controller re-derive the reason from node_id.

No destroy link added to nodes#show's events list - deliberate, per
existing subnav-semantics convention (destructive actions live on the
resource's own views). Covered directly by test.

Adds real coverage for EventsController, previously 100% commented-out
scaffold, plus unit tests for the new tag-mapping helper and the
weekday-abbreviation helpers from the prior commit.

1 files changed, 15 insertions, 0 deletions

diff --git a/test/models/concerns/rrule_humanizer_test.rb b/test/models/concerns/rrule_humanizer_test.rb
index 500dbc7..279ff73 100644
--- a/test/models/concerns/rrule_humanizer_test.rb
+++ b/test/models/concerns/rrule_humanizer_test.rb
@@ -81,4 +81,19 @@ class RruleHumanizerTest < ActiveSupport::TestCase
   test "falls back to english for unknown locale" do
     assert_equal "Every Tuesday", humanize("FREQ=WEEKLY;BYDAY=TU", :fr)
   end
+
+  test "wday_abbr returns the correct German abbreviation for each day" do
+    monday = Time.parse("2026-07-06") # confirmed Monday
+    assert_equal "Mo", RruleHumanizer.wday_abbr(monday, :de)
+    assert_equal "Di", RruleHumanizer.wday_abbr(monday + 1.day, :de)
+    assert_equal "Mi", RruleHumanizer.wday_abbr(monday + 2.days, :de)
+    assert_equal "Do", RruleHumanizer.wday_abbr(monday + 3.days, :de)
+    assert_equal "Fr", RruleHumanizer.wday_abbr(monday + 4.days, :de)
+    assert_equal "Sa", RruleHumanizer.wday_abbr(monday + 5.days, :de)
+    assert_equal "So", RruleHumanizer.wday_abbr(monday + 6.days, :de)
+  end
+
+  test "wday_abbr falls back to :de for an unrecognized locale" do
+    assert_equal "Mo", RruleHumanizer.wday_abbr(Time.parse("2026-07-06"), :fr)
+  end
 end